Method for conveying hydraulic fluid, and electrohydraulic motor-pump unit therefor

ABSTRACT

An electrohydraulic motor-pump unit comprises an unsteadily conveying displacement pump for conveying hydraulic fluid in a hydraulic system, an electric motor coupled with the displacement pump for driving the displacement pump, and an electrical control device coupled with the electric motor for controlling the electric motor. By way of a detector there is captured a pulsation parameter, which originates from a pulsation arising due to unsteady conveyance of hydraulic fluid in the hydraulic system, and by way of a modulator as part of the electrical control device the drive torque or the rotating speed of the electric motor for driving the displacement pump is modulated on the basis of the captured pulsation parameter. As a result the drive torque or the rotating speed of the electric motor is modulated in accordance with the conveying frequency of the displacement pump.

The present invention relates to a method for conveying hydraulic fluid in a hydraulic system by means of an electrically driven, unsteadily conveying displacement pump or, more precisely, a method for smoothing a pulsation of the hydraulic fluid arising in the hydraulic system due to the unsteady conveyance. The invention relates furthermore to an electrohydraulic motor-pump unit for employment in the method according to the invention.

The term “electrohydraulic motor-pump unit” refers to a system consisting of a hydraulic displacement unit, an electric motor and an appurtenant electrical control device. Such motor-pump units are frequently designated “power packs” colloquially. They serve for converting electrical energy to hydraulic energy and find use in numerous areas of application, such as the automobile sector, in mobile work machines or also quite generally in the industrial sector.

Different hydraulic displacement units, electric motors and electrical controls for the electric motors are known in connection with such motor-pump units. A great variety of principles of displacement are used here, such as for example internal gear pumps, external gear pumps, piston pumps, and the like. All these principles of displacement have in common that the displacement pumps are characterized by an unsteady conveyance of the hydraulic fluid, in spite of a steady drive by means of the electric motor, which results in a more or less pronounced pulsation of the pressure in the hydraulic connections, both on the suction side and—in particular—on the pressure side.

The unsteadiness of the hydraulic fluid conveyance is due to the fact that, depending on their design, the displacement pumps have one or more pump chambers which successively first draw in hydraulic fluid from the suction side and subsequently discharge it on the pressure side. Thus, for example in piston pumps, a pressure pulse is generated in the hydraulic system with every stroke. In the case of gear pumps, a number of pressure pulses corresponding to the number of pump gear teeth is generated in the hydraulic system with every rotation of the pump gear. The pulsation frequency thus depends on the speed at which the displacement pump is operated, that is, on its “conveying frequency”, i.e. the frequency per time unit at which the displacement pump draws in and discharges hydraulic fluid by means of its pump chamber or pump chambers. The conveying frequency is generally regular, but this is by no means necessary.

The pressure pulsation occurring in the hydraulic fluid as a result of the unsteady conveying volume leads to disadvantageous phenomena in the hydraulic system, in particular frequently bothersome noise as a result of excited vibrations.

To avoid the pressure pulsation being transferred to the hydraulic system, secondary measures are taken in known hydraulic systems, with for example hydraulic accumulators, so-called pulsation dampeners, or also baffles being integrated in the lines, or the lines being formed at least partly by hoses with defined resilience, so-called extension hoses.

Depending on the respective smoothing measures, they involve effort and/or costs in selection, coordination, arrangement and/or assembly.

The object of the present invention is to provide an alternative for smoothing the pressure pulsation in a hydraulic system that occurs due to the unsteady conveyance by means of displacement pumps. In this connection it is a further object of the present invention to propose an electrohydraulic motor-pump unit suitable therefor.

Accordingly, a method according to the invention for conveying hydraulic fluid in a hydraulic system by means of an electrically driven, unsteadily conveying displacement pump provides for smoothing the pulsation of the hydraulic fluid arising in the hydraulic system due to the unsteady conveyance by suitably modulating the drive torque or the rotating speed of the electrical drive in accordance with the conveying frequency of the displacement pump.

Since the pulsation in the hydraulic system depends directly on the conveying frequency of the displacement pump, there can advantageously be captured and utilized for the purposes of said modulation a pulsation parameter characterizing the pulsation, for example i.e. the pressure pulsation in the hydraulic system, and the drive torque of the electrical drive for driving the displacement pump can be modulated in accordance with said captured pulsation parameter.

For the pulsation in the hydraulic system caused by the pump chambers of the displacement pump generates an associated torque pulsation on the drive shaft of the displacement pump. Via the corresponding modulation of the drive torque of the electrical drive on the basis of the captured pulsation parameter, i.e. for example on the basis of the pressure pulsation captured in the hydraulic system, it is possible to compensate the torque unsteadiness on the drive shaft of the displacement pump and thus already smooth, that is, eliminate or at least minimize, the pulsation on the hydraulic connections.

Instead of modulating the drive torque of the electrical drive, it is alternatively also possible to directly modulate the rotating speed of the electrical drive. For the modulation of the drive torque also ultimately leads to nothing other than a corresponding change, i.e. modulation, of the rotating speed and hence as a result respectively to a temporal modulation of the conveyance through the displacement pump.

If the pressure pulsation in the hydraulic system is used as a hydraulic parameter, it is advantageous to capture the pressure on one or more of the hydraulic connections of the motor-pump unit for example by means of a pressure sensor, preferably on the pressure side. Alternatively, instead of the pressure, there can also be captured the pulsation of the volume flow, preferably again directly on one or more of the hydraulic connections of the motor-pump unit, and preferably again on the pressure side.

The captured pulsation parameter, whether it be the pulsation of the hydraulic pressure or the pulsation of the conveyed volume flow, then serves as the input variable for the control device of the electric motor for balancing out the pulsation, after the captured pulsation parameter has been resolved into a signal processable by the control device. Since said input variable reacts upon the input variable again via the control device of the electric motor and via the displacement pump driven by means of the electric motor, the overall result is a feedback control system.

According to a preferred embodiment variant, there is used as a pulsation parameter, instead of the pressure pulsation or the volume flow pulsation, a pulsation or vibration of the drive torque of the electric motor itself. For, as mentioned hereinabove, the pulsation caused by the unsteadily conveying displacement pump generates an associated torque pulsation on the drive shaft of the displacement pump. Through a circuit, implemented in the electrical control device, for establishing the torque of the electric motor, said torque pulsation of the drive shaft can be captured and used for modulating the drive torque of the electric motor for driving the displacement pump.

For the necessary modulation of the drive torque on the basis of such a pulsation parameter, an accordingly fast-reacting electrical control device of the electric motor is required. Suitable for such extremely fast cycle times in electronics are for example circuits with so-called field programmable gate arrays (FPGAs). Further, when selecting the electric motor one should make sure the model has sufficient dynamics.

An electrohydraulic motor-pump unit suitable for the hereinabove described method accordingly comprises an unsteadily conveying displacement pump for conveying hydraulic fluid in a hydraulic system, an electric motor coupled with the displacement pump for driving the displacement pump, and an electrical control device coupled with the electric motor for controlling the electric motor, wherein the displacement pump, the electric motor and the control device are preferably housed in a common housing block having hydraulic connections which are arranged for coupling the motor-pump unit to a hydraulic system. The housing block can furthermore advantageously comprise a hydraulic fluid tank for the hydraulic system. In any case the electrohydraulic motor-pump unit additionally has, as part of the electrical control device, a modulator for modulating the drive torque of the electric motor in accordance with the conveying frequency of the displacement pump, wherein preferably furthermore a detector for capturing one of the hereinabove described pulsation parameters is provided and the modulator is arranged for modulating the drive torque of the electric motor on the basis of the captured pulsation parameter.

However, the capturing of a pulsation parameter and modulating of the drive torque on the basis of the captured pulsation parameter, that is, the setup of a complete feedback control system, is by no means necessary for smoothing the pulsation. A smoothing is already obtained when the modulation of the drive torque in accordance with the conveying frequency of the displacement pump is firmly preset on the basis of values derived from experience. It is true that the pulsation also depends on properties of the hydraulic system, in particular the elasticity of the hydraulic system, so that a completely feedback-controlled system with consideration of current pulsation parameters is advantageous. But a firm presetting of a drive-torque modulation dependent on the conveying frequency of the displacement pump can already lead to satisfactory results. For purposes of optimization it is also possible to adapt said firm presetting later, or to only set it when the motor-pump unit is connected to the hydraulic system for which it is intended.

The advantages achieved with the invention consist in that the pressure pulsation on the hydraulic connections can be minimized or almost completely eliminated. Secondary measures for pulsation reduction in the hydraulic system can thereby be omitted, where applicable, so that for example assembly effort and/or costs can be reduced.

Hereinafter the invention will be described by way of example with reference to the accompanying drawings. Therein are shown:

FIG. 1 a realistic representation of an electrohydraulic motor-pump unit according to a first exemplary embodiment of the invention on a 1:1 scale,

FIG. 2 a schematic and idealized representation of the time course of the hydraulic pressure of the drive torque and of the motor speed in a hydraulic system with and without pulsation compensation,

FIG. 3 a realistic representation of an electrohydraulic motor-pump unit according to a second exemplary embodiment of the invention on a 1:1 scale, and

FIG. 4 a realistic representation of an electrohydraulic motor-pump unit with an integrated hydraulic tank on a 1:2 scale.

FIG. 1 shows a first exemplary embodiment of an electrohydraulic motor-pump unit, wherein a displacement pump 1, an electric motor 2 and an electrical control device 3 are housed in a common housing block. The displacement pump 1 possesses two hydraulic connections 4 for connecting the motor-pump unit to the suction side and the pressure side of a hydraulic system. Further hydraulic connections can be provided.

As displacement pumps there come into consideration a great variety of unsteadily conveying hydraulic displacement units, such as for example the hereinabove mentioned internal gear pumps, external gear pumps, piston pumps or other pumps with successively conveying pump chambers. The nature of the electric motor is substantially uncritical for the invention. What is crucial is that the drive torque for the electric motor 1 supplied by the electric motor and the rotating speed of the electric motor 1 are adjustable, for drive torque and rotating speed are directly related to each other. For adjusting or modulating the drive torque or the rotating speed of the electric motor 2 there is used the electrical control device 3.

A pressure sensor 5 is so disposed on the hydraulic pump 1 that the pressure on the pressure-side hydraulic connection 4 can be measured therewith. In reversible motor-pump units it may be expedient to provide a further pressure sensor on the second hydraulic connection 4 in a corresponding manner. Via a return line 6 the sensor signal supplied by the pressure sensor 5 is fed to the electronic control device 3. The sensor signal is processed in the electronic control device 3 and employed for modulating the drive torque of the electric motor in accordance with the conveying frequency of the displacement pump such that it is constant if possible. As a result this also leads to the rotating speed of the electric motor being modulated. Through suitable modulation of the drive torque or of the rotating speed of the electric motor the pressure relations in the hydraulic system can be changed, and through suitable change pressure fluctuations can be compensated. Since the pressure sensor 5 captures pressure fluctuations directly on the hydraulic connection 4 and since the pressure fluctuations occurring on the hydraulic connection originate substantially solely from the unsteady conveyance of the hydraulic fluid by means of the displacement pump, the described setup of the motor-pump unit according to FIG. 1 enables pressure pulsations in the hydraulic system to be smoothed through suitable modulation of the drive torque or of the rotating speed of the electric motor.

FIG. 2 shows schematically over the time t the course of the pressure p in the hydraulic system and the course of the drive torque M on the drive shaft of the electrical drive in comparison to the course of the rotating speed (U/min, or rpm) of the electrical drive. The course without pressure compensation control is represented therein in dash lines with p₀, M₀ and U₀/min, while the course with compensation control is respectively represented as a continuous line. In this idealized representation it is recognizable that upon operation without compensation control the pressure p₀ in the hydraulic system and the drive torque of the electrical drive respectively fluctuate around an average p_(M) and M_(M), while the motor speed remains constant at an average rotational speed U_(M)/min. In reality, said value U_(M)/min actually also fluctuates around the average U_(M)/min, but only slightly, because the motor conveys alternately against slightly higher and lower pressures in the hydraulic system due to the unsteady conveyance and the resulting volume flow pulsation.

By the drive torque M₀ of the electrical drive now being modulated in accordance with the hydraulic pressure measured on the hydraulic connection 4, a change of the rotating speed p₀ of the electric motor is obtained, the result being that the time course of the hydraulic fluid volume conveyed by the displacement pump undergoes a corresponding change, so that ultimately the hydraulic pressure p present in the hydraulic system and on the hydraulic connections 4 and the drive torque M present on the electric motor also change accordingly. The modulation of the drive torque of the electric motor or of the rotating speed of the electric motor is in so doing adjusted such that the pressure pulsation on the hydraulic connection 4 is ideally compensated completely. That is to say, in an idealized view, at times when the pressure p₀ in the system sinks due to the pressure pulsation, the drive torque M or the rotating speed of the electric motor is so modulated that the electric motor conveys more volume per time unit in order to raise the lower system pressure p₀ to the average pressure p_(M), and at a higher system pressure p₀ vice versa.

FIG. 3 shows a second exemplary embodiment of an electrohydraulic motor-pump unit. In this exemplary embodiment, a different pulsation parameter is used for modulating the drive torque or the rotating speed of the electric motor 2, i.e. the drive torque M₀ of the electric motor 2 is utilized as a basis for the modulation instead of the hydraulic pressure p₀ in the hydraulic system. That is to say, by means of an evaluation circuit of the electrical control device 3 it is monitored to what extent the drive torque M₀ of the electric motor 2 pulsates due to the unsteadily conveying displacement pump 1, and this pulsation parameter serves in the electrical control device 3 for regulating the drive torque or the rotating speed of the electric motor 2 such that all drive torque fluctuations are compensated if possible. This technical solution is the least elaborate constructively and leads to a smoothing of the pressure pulsation in the hydraulic system in a simple manner, because all pressure pulsation in the hydraulic system reacts upon the torque Mo present on the electric motor 2.

FIG. 4 shows a motor-pump unit, having a hydraulic displacement unit 1, an electric motor 2 and an integrated hydraulic tank 7, as is preferably employed in a hydraulic system. The hydraulic lines are marked as 8. The electrical control device 3 is not clearly recognizable here, but is part of the electric motor 2.

In the above-described pulsation compensation circuits an accordingly fast electrical control of the electric motor 1 is required due to the extremely fast cycle times. Circuits with field programmable gate arrays (FPGAs) are advantageously suited therefor, whereby when selecting the electric motor 1 one should of course also make sure the model has sufficient dynamics. 

1. An electrohydraulic motor-pump unit, comprising: an unsteadily conveying displacement pump which is arranged for conveying hydraulic fluid in a hydraulic system, an electric motor coupled with the displacement pump and arranged for driving the displacement pump, and an electrical control device coupled with the electric motor and arranged for controlling the electric motor, further comprising a modulator as part of the electrical control device, which is arranged for modulating a drive torque or a rotating speed of the electric motor for driving the displacement pump in accordance with a conveying frequency of the displacement pump.
 2. The motor-pump unit according to claim 1, wherein the displacement pump, the electric motor and the electrical control device are housed in a common housing block having hydraulic connections which are arranged for coupling the motor-pump unit to a hydraulic system, wherein the housing block preferably further comprises a hydraulic fluid tank for hydraulic fluid.
 3. The motor-pump unit according to claim 1, comprising a detector which is arranged for capturing a pulsation parameter which originates from a pulsation arising due to unsteady conveyance of hydraulic fluid in a hydraulic system, wherein the modulator is arranged for modulating the drive torque or a rotational speed of the electric motor on the basis of the captured pulsation parameter.
 4. The motor-pump unit according to claim 3, further comprising, as a detector, a pressure sensor which is arranged for capturing as the pulsation parameter a hydraulic pressure.
 5. The motor-pump unit according to claim 3, further comprising, as a detector, a pressure sensor on one of the hydraulic connections, which is arranged for capturing as the pulsation parameter a hydraulic pressure present on the hydraulic connection.
 6. The motor-pump unit according to claim 3, further comprising, as a detector, an evaluation circuit which is arranged for capturing as the pulsation parameter the drive torque of the electric motor, wherein the evaluation circuit is implemented in the electrical control device.
 7. The motor-pump unit according to claim 1, wherein the modulator comprises a circuit with an FPGA (field programmable gate array).
 8. A hydraulic system, comprising hydraulic lines and a motor-pump unit according to claim 1 connected to said hydraulic lines.
 9. A method for conveying hydraulic fluid in a hydraulic system by an electrically driven, unsteadily conveying displacement pump, comprising: smoothing a pulsation of the hydraulic fluid in the hydraulic system, arising on due to the unsteady conveyance, by modulating a drive torque or a rotating speed of the electrical drive in accordance with a conveying frequency of the displacement pump.
 10. The method according to claim 9, wherein smoothing comprises a pulsation parameter wherein the pulsation is captured and the drive torque or the rotational speed of the electrical drive is modulated on the basis of the captured pulsation parameter.
 11. The method according to claim 10, wherein the drive torque or the rotational speed of the electrical drive is modulated on the basis of a pressure pulsation measured in the hydraulic fluid.
 12. The method according to claim 10, wherein the drive torque of the electrical drive is modulated on the basis of a torque pulsation measured on the electrical drive.
 13. The method according to claim 10, wherein the modulating of the drive torque or of the rotational speed of the electrical drive is effected via a circuit with an FPGA (field programmable gate array).
 14. The method according to claim 9, further comprising employing a motor-pump unit. 